High Performance Bulky α-Diimine Nickel(II) Catalysts for Ethylene Polymerization

Bulky aryl α-diimine nickel(II) complexes with hydroxy functionality at the para-aryl position based on acenaphthenequinone with high efficiency were synthesized and characterized. These hydroxy functionalized ligands with different substitutes on their 2,6-ortho-aryl position including methyl, isopropyl, phenyl, 4-tbutyl-phenyl were used in ethylene polymerization. Ethylene polymerization wascarried out using α-diimine nickel(II) complexes at different concentrations of ethyl aluminium sesquichloride (EASC) as an efficient cocatalyst. The effects of polymerization temperature from 10°C to 60°C and hydrogen concentration on catalysts performance were also investigated. The highest activity was achieved at 45°C and the activity was decreased at higher temperature for catalysts b and d the same as b . The behaviours of these complexes were also compared with the same catalysts without hydroxy functionality. As expected, the α-diimine catalyst bis[N,N -(4-(3-hydroxypropyl)-2,6-di((4-t-butyl-phenyl)-phenyl))imino]acenaphthene nickel(II) dibromide (d) and bis[N,N -2,6-di((4-t-butyl-phenyl)-phenyl)imino]acenaphthene (d ) had higher yield and molecular weight because of higher steric blocking of metal axial sites which leads to higher insertion rate of monomer and retardation of chain transfer. These catalysts polymerized ethylene in 45°C and 10 bar monomer pressure with exceptional activity up to 30.1×106 g PE/mol Ni.h. In the case of 2,6-di-(4-t-butyl phenyl) aryl α-diiminenickel(II) catalysts, the molecular weight increased up to 3.9×106 g/mol with polydispersity index of 4.74. Despite the behaviour of Brookhart catalysts, this α-diimine catalyst d did not show any significant drop in its activity in presence of hydrogen which was added to polymerization reaction to control polymer molecular weight.